Many industrial processes spray coating compositions that contain viscous or solid polymeric components, such as coatings, adhesives, release agents, gel coats, lubricants, and agricultural materials. To spray such materials, it has been common practice to use relatively large amounts of organic solvents. The solvents perform a variety of functions, such as to dissolve the polymers; to reduce viscosity for spraying; to provide a carrier medium for dispersions; and to give proper flow when the composition is sprayed onto a substrate, such as coalescence and leveling to form a smooth coherent coating film. However, the solvents released by the spray operation are a major source of air pollution.
There are several patents which disclose new spray technology that can markedly reduce organic solvent emissions, by using environmentally acceptable supercritical fluids or subcritical compressed fluids, such as carbon dioxide, to replace the solvent fraction in solvent-borne compositions that is needed to obtain low spray viscosity: U.S. Pat. Nos. 4,923,720 and 5,108,799 disclose methods for using supercritical fluids for the spray application of coatings. U.S. Pat. No. 5,106,650 discloses methods for using supercritical carbon dioxide for the electrostatic spray application of coatings. U.S. Pat. No. 5,009,367 discloses methods for using supercritical fluids for obtaining wider airless sprays. U.S. Pat. No. 5,057,342 discloses methods for using supercritical fluids for obtaining feathered airless sprays. U.S. Pat. No. 4,882,107 discloses methods for using supercritical fluids to apply mold release agents, such as in the production of polyurethane foam. U.S. Pat. No. 5,066,522 discloses methods for using supercritical fluids to apply adhesive coatings.
The conventional atomization mechanism of airless sprays is well known and is discussed and illustrated by Dombroski, et al., Chemical Engineering Science 18: 203 (1963). The coating exits the orifice as a liquid film that becomes unstable from shear induced by its high velocity relative to the surrounding air. Waves grow in the liquid film, become unstable, and break up into liquid filaments that likewise become unstable and break up into droplets. Atomization occurs because cohesion and surface tension forces, which hold the liquid together, are overcome by shear and fluid inertia forces, which break it apart. This process is shown photographically for an actual paint in the brochure entitled "Cross-Cut.TM. Airless Spray Gun Nozzles", Nordson Corporation, Amherst, Ohio. Often the liquid film extends far enough from the orifice to be visible before atomizing into droplets. The sprays are generally angular in shape and have a relatively narrow fan width, that is, a fan width that is not much greater than the fan width rating of the spray tip being used. Viscous dissipation markedly reduces atomization energy, so relatively coarse atomization typically results. As used herein, the terms "liquid-film spray" and "liquid-film atomization" are understood to mean a spray, spray fan, or spray pattern in which atomization occurs by this conventional mechanism.
Liquid-film sprays characteristically form a "tailing" or "fishtail" spray pattern, wherein coating material is distributed unevenly in the spray. Surface tension often gathers more liquid at the edges of the spray fan than in the center, which can produce coarsely atomized jets of coating that sometimes separate from the spray. At other times the edges of the spray are thickened so that more coating is deposited at the top and bottom than in the center of the spray. These deficiencies produce a nonuniform deposition pattern that makes it difficult to apply a uniform coating.
Examples of liquid-film sprays are shown photographically in the aforementioned commonly assigned patents, namely, in FIGS. 4a, 4b, 4c, 4d, 10a, 11a, 12a, and 12b of U.S. Pat. No. 5,057,342 and in FIGS. 3a, 3b, 3c, 9a, 9b, and 9c in U.S. Pat. No. 5,009,367.
As disclosed in the aforementioned patents, supercritical fluids or subcritical compressed fluids such as carbon dioxide or ethane are not only effective viscosity reducers, they can produce a new airless spray atomization mechanism, which can produce finer droplet size than by conventional airless spray methods and a feathered spray needed to apply high quality coatings. Without wishing to be bound by theory, the new type of atomization is believed to be produced by the dissolved carbon dioxide suddenly becoming exceedingly supersaturated as the spray mixture experiences a sudden and large drop in pressure in the spray orifice. This creates a very large driving force for gasification of the carbon dioxide. The carbon dioxide gas released from solution during depressurization expands in volume and produces an expansive force that overwhelms the cohesion, surface tension, and viscosity forces that oppose atomization and normally bind the fluid flow together.
A different atomization mechanism is evident because atomization appears to occur right at the spray orifice instead of away from it. Atomization is believed to be due not to break-up of a liquid film from shear with the surrounding air but, instead, to the force of the expanding carbon dioxide gas. Therefore, no liquid film is visible coming out of the nozzle.
Furthermore, because the spray is no longer bound by cohesion and surface tension forces, it typically leaves the nozzle at a much wider angle than normal airless sprays and produces a "feathered" spray with tapered edges like an air spray. This typically produces a rounded, parabolic-shaped spray fan, instead of the sharp angular fans typical of conventional airless sprays. The spray also typically has a much wider fan width than conventional airless sprays produced by the same spray tip. As used herein, the terms "decompressive spray" and "decompressive atomization" are understood to mean to a spray, spray fan, or spray pattern that has the preceding characteristics.
Examples of decompressive sprays are shown photographically in the aforementioned patents, namely, in FIGS. 3a, 3b, 3c, 3d, 3e, 10b, 11b, 12c, 12d, and 13 of U.S. Pat. No. 5,057,342 and in FIGS. 4b, 4c, 8, and 9d of U.S. Pat. No. 5,009,367.
Laser light scattering measurements and comparative spray tests show that this decompressive atomization can produce fine droplets that are in the same size range as air spray systems, instead of the relatively coarse droplets produced by liquid-film airless sprays. For a properly formulated coating composition, the droplet size range and distribution are ideal for minimizing orange peel and other surface defects commonly associated with spray application. This fine particle size provides ample surface area for the dissolved carbon dioxide to very rapidly diffuse from the droplets within a short distance from the spray nozzle. Therefore, the coating is essentially free of carbon dioxide before it is deposited onto the substrate.
A liquid-film spray can undergo a transition to a decompressive spray as the concentration of supercritical fluid or subcritical compressed fluid such as carbon dioxide is increased. The transition can also occur as the temperature is increased, for suitable concentrations. The transition has been found to occur over a relatively narrow range of concentration or temperature. As the carbon dioxide concentration is increased, the liquid-film spray at first remains generally angular in shape, has a relatively constant or slightly increased width that is characteristic of the width obtained when the composition is sprayed with no carbon dioxide, and has a relatively large average droplet size. A visible liquid film can typically be seen to recede towards the orifice. Atomization occurs predominately due to instability induced by shear with the surrounding air. The spray pattern is controlled predominately by the cohesion, viscosity, and surface tension forces. The boundary of the liquid-film region typically occurs about at the carbon dioxide concentration at which the liquid film disappears. As the concentration increases, the spray then passes through a transition region in which the spray pattern typically undergoes dramatic changes, which depend upon the coating composition, as it transforms from a liquid-film to a decompressive spray and the atomization mechanism changes. The shape and width of the transition spray typically changes markedly for relatively small changes in carbon dioxide concentration. For some coating compositions, the spray pattern collapses from a flat fan into a narrower, irregular, conical spray and then expands into a wider, flat, parabolic decompressive spray. Sometimes the spray collapses completely into a single round jet, or into two, three, or more jets spaced at irregular angles, before expanding into a decompressive spray. For other coating compositions, the spray pattern remains mostly planar but the center flares outward, more as the spray narrows and then less as the spray expands into a decompressive spray. Sometimes the spray remains planar as a decompressive spray pattern forms superimposed upon the liquid-film spray pattern, which simultaneously disappears. For still other coating compositions, the angular spray pattern first becomes much wider and then changes to a parabolic shape. The transition sprays are irregular and often unstable because neither the expansive force from the release of the gaseous carbon dioxide nor the cohesion, viscosity, and surface tension forces of the coating composition dominate the atomization and spray pattern formation. The different types of spray transitions are due to different surface tension and rheological properties of different coating compositions. A decompressive spray forms when the carbon dioxide concentration becomes high enough for the expansive force of the gaseous carbon dioxide to overcome the cohesion, viscosity, and surface tension forces of the coating composition in the forming spray. The decompressive spray that forms is generally substantially planar, mostly parabolic in shape, and significantly wider than the corresponding liquid-film spray. Near the spray boundary, the decompressive spray may have some jetting or be somewhat flared at the center of the spray, and the spray pattern may be fingered. However, these typically dissipate and the spray pattern becomes more uniform at higher compressed fluid concentration. The planar decompressive spray, in addition to being wider, is also characteristically thicker across the plane of the spray than the corresponding liquid-film spray. One of the characteristics of the transition from a liquid-film to a decompressive spray is a marked decrease in the average droplet size of the spray.
U.S. Pat. No. 5,057,342 provides examples of the transition from a liquid-film to a decompressive spray for supercritical carbon dioxide. FIGS. 12a to 12d show the transition for a thermosetting acrylic coating composition at a temperature of 60 Celsius and a pressure of 1600 psig. FIG. 12a shows a liquid-film spray with 14 percent carbon dioxide (by weight). The liquid film can be seen jetting from the orifice. FIG. 12b illustrates a liquid-film spray at the boundary of the transition region, which occurs with about 19.7 percent carbon dioxide. The liquid film has disappeared but the spray is still significantly angular in shape. FIG. 12c shows a decompressive spray that forms with about 22 percent carbon dioxide, which is close to the transition region. The spray flares outward somewhat from the plane of the spray at the center. The angle at which the spray emerges from the orifice is much larger than for the liquid-film spray. FIG. 12d shows the uniform decompressive spray that forms with about 25 percent carbon dioxide.
Generally, the preferred upper limit of supercritical fluid or compressed fluid addition, such as carbon dioxide, is that which is capable of being miscible with the polymeric coating composition. This practical upper limit is generally recognizable when the admixture containing coating composition and carbon dioxide breaks down from one phase into two fluid phases. Spraying significantly inside the two phase region is avoided, because a significant amount of organic solvent is typically extracted from the liquid polymer phase into the liquid carbon dioxide phase, which can significantly increase the viscosity of the liquid spray mixture. This increases the droplet size and causes the spray pattern to deteriorate. Furthermore, for coating applications, film formation usually deteriorates, because the solvent loss causes the deposited coating material to be too viscous to flow out properly on the substrate.
Because of increased environmental concern about the emissions of solvent from spray coating operations, water-borne coatings have also been developed, wherein water is used to achieve low atomization viscosity instead of the fast-evaporating organic solvents used in solvent-borne coatings. Water-borne coatings have found widespread use for applications having substrates that tolerate water. However, water-borne coatings have generally not provided the performance and application properties that were initially expected. Water has a relatively low evaporation rate when compared to fast evaporating solvents used in solvent-borne coatings, and it is much slower evaporating than supercritical fluids such as carbon dioxide. One difficult problem is that often an insufficient amount of water evaporates from the spray, so a desirable high deposition viscosity is not achieved, unlike solvent-borne coatings and coatings applied with supercritical fluids. Therefore, water-borne coatings have persistent problems with runs and sags occurring in the applied coating, because of low deposition viscosity. Another difficult problem is that line speeds on water-borne coating spray lines are often significantly slower than for solvent-borne coatings, which significantly reduces productivity, because water evaporates more slowly from the applied coating than comparable organic solvents. These problems are made more difficult by the water evaporation rate being sensitive to the relative humidity of the spray environment. Water-borne coatings generally can not be applied under conditions of high relative humidity without serious coating defects. These defects arise when the water evaporates more slowly than the organic cosolvents of the coalescing aid, and as might be expected in the case of aqueous dispersions, the loss of the organic cosolvent/coalescing aid interferes with film formation. Poor gloss, poor uniformity, and pin holes often result.
Therefore, there is clearly a need for an improved process by which water-borne coatings can be sprayed at lower water levels and higher viscosities. Such a process would give: improved coating quality by decreasing coating sagging and running, because less water would need be evaporated in the spray; improved productivity and higher line speed, because less water would need to be evaporated from the applied coating; and less sensitivity to relative humidity.
Prior to the present invention, it was unknown how a supercritical fluid or subcritical compressed fluid such as carbon dioxide would interact with a water-borne coating composition. As disclosed in the aforementioned U.S. Pat. Nos. 5,009,367 and 5,057,342, generally the coating compositions used for spray application by supercritical fluids may contain up to about 30 percent by weight of water, preferably up to about 20 percent, in the solvent fraction provided that a coupling solvent is preferably also present in the composition. A coupling solvent is a solvent in which the polymers in the composition are at least partially soluble and which is at least partially miscible with water. The coupling solvent enables mutual solubility and miscibility of the polymers, solvents, and water within a single liquid phase. As further disclosed in Australian Patent No. 630170, issued Feb. 12, 1993, the solubility of supercritical carbon dioxide in coating compositions that contain a coupling solvent and up to about 30 percent water by weight of the solvent fraction is often substantially the same as that for the coating composition containing no water. A higher quantity of water, however, was generally found not to be desirable. Too much water could result in phase separation, that is, the composition could break down into a water phase and an organic phase, which could cause poor spraying and coating performance. Higher levels of water could also significantly reduce the solubility of the supercritical carbon dioxide in the coating composition and therefore the amount of supercritical carbon dioxide diluent that could be used for spraying. Phase separation caused by excess supercritical carbon dioxide was found to give generally poor spray and coating performance and therefore was avoided. It was believed that this reduced level of dissolved carbon dioxide would give insufficient viscosity reduction of the viscous coating composition and also insufficient expansive force to obtain a decompressive spray needed to apply high quality coatings. Therefore, it was generally believed that the much lower solubility of supercritical carbon dioxide in water-borne coatings, even with the use of coupling solvents, would preclude spray application of water-borne coatings with supercritical fluids or subcritical compressed fluids like carbon dioxide.